There is a growing demand in the mobile communication area for a system having the ability to download large blocks of data to a Mobile Station (MS) on demand at a reasonable rate. Such data could for example be web pages from the Internet, possibly including video clips or similar. Typically a particular MS will only require such data intermittently, so fixed bandwidth dedicated links are not appropriate. To meet this requirement in UMTS, a High-Speed Downlink Packet Access (HSDPA) scheme is being developed which may facilitate transfer of packet data to a mobile station at up to at least 4 Mbps.
A conventional component of a packet data transmission system is an ARQ (Automatic Repeat reQuest) process, for handling data packets received in error. For example, consider downlink packet transmission from a Base Station (BS) to a Mobile Station (MS) in HSDPA. When the MS receives a data packet it determines whether the packet has been corrupted, for example using Cyclic Redundancy Check (CRC) information. It then transmits a signal in a field allocated for this purpose to the BS, with a first signal used as an acknowledgement (ACK), to indicate that the packet was successfully received, and a second signal used as a negative acknowledgement (NACK), to indicate that the packet was received but corrupted. The signals may for example be different codewords or the same codeword transmitted at different powers.
In the presently-defined scheme for HSDPA a number of channels are defined, each handling different types of information: only those relevant to the present invention will be discussed here. Data packets are transmitted from the BS to the MS on a (high speed) downlink data channel, and in the reverse direction on an uplink data channel. Two uplink control channels are defined for signalling by the mobile. Transmission of ACK/NACK messages is performed on a first uplink control channel (a High Speed Dedicated Physical Control CHannel, HS-DPCCH), while transmission of pilot information, to enable the BS to obtain a channel estimate of the uplink channel, is performed on a second uplink control channel (a DPCCH). The power level of the second control channel is set by the BS using closed loop power control. The power level of the uplink data channel is determined from the power of the second control channel. The ratio of these two power levels is determined by a gain factor which may be signalled to the MS or determined independently by the MS.
The consequences of errors in ACK and NACK messages are significantly different. Normally the BS would retransmit a packet if a NACK were received. If the BS receives a NACK when an ACK is sent, then the packet is re-transmitted anyway. This only wastes a little system resource. If a NACK is sent, but received as an ACK, then no re-transmission is sent. Without special physical layer mechanisms, this situation can only be recovered from by using higher layer processes, which add delay and represent a significant waste of system resources. Thus the cost of an error in a NACK is much more serious than the cost of an error in a ACK, and consequently the performance requirements in 3GPP have been set at 10−2 for ACK errors and 10−4 for NACK errors. Achieving these error rates require good channel estimates. In particular, the channel phase is needed to determine correctly the phase of data symbols when phase modulation is used (for example BPSK or QPSK). For a modulation scheme where amplitude is also significant, such as m-QAM (where m may for example be 16), the channel amplitude would also be needed.